What is the best over-the-counter product to relieve neck pain?

NSAIDs. Nonsteroidal anti-inflammatory drugs (NSAIDS) can be excellent for this type of pain. They can help with both pain relief and in reducing inflammation. Examples are ibuprofen (like Advil, Nuprin or Motrin), Naproxen sodium (like Aleve, (naproxen) Naprosyn, Anaprox) or Asprin (such as Bayer, Ecotrin, Ascriptin). Avoid NSAID's if pregnant or allergic to them.
Anti-inflammatories. As long as you no specific health reasons not to take them, over the counter anti-inflammatories such as ibuprofen, naproxen, advil, etc are typically very helpful to treat acute problems, consider discussing with your primary care physician to make sure you are taking the right dose.